1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method for controlling power-off of a mobile station.
2. Description of the Related Art
Unlike the public switched telephone network (PSTN), a public land mobile network (PLMN) manages the connection between a mobile station (MS) and a base station (BS) in a radio environment.
A 3G (3rd Generation) IS-95 mobile communication system capable of transmitting not only voice but also high-speed data, can provide a high-quality voice service, a moving image service, and an Internet search service. In such a mobile communication system, a radio communication link existing between the MS and the BS is divided into a forward link transmitted from the BS to the MS and a reverse link transmitted from the MS to the BS. For an exchange of signaling data between the BS and the MS before setup of a voice call, a paging channel which is a forward common channel and an access channel which is a reverse common channel are used. When transmitting a message from the BS to the MS, the BS transmits the message over the paging channel and receives a response or ACK (acknowledge) from the MS over the access channel. To the contrary, when transmitting a message from the MS to the BS, the MS transmits the message over the access channel and receives a response from the BS over the paging channel. Therefore, there may exist a plurality of the paging channels and the access channels: the paging channels are distinguished by unique Walsh codes, while the access channels are distinguished by unique long codes.
In the IS-95A, IS-95B or J-STD0008 mobile communication system, when an MS user makes a power-off request using a power key, the MS attempts to send a power-down registration request to the BS as many times as a predetermined maximum attempt sequence (max_probe_sequence or max_attempt_sequence) using the access channel. Upon failure to receive a response to the power-down registration request from the BS, the MS continuously attempts to send the power-down registration request as many times as up to the predetermined maximum attempt sequence (max_probe_sequence or max_attempt_sequence) while increasing transmission power little by little. Upon receipt of a power-down registration confirm response from the BS in the process of sending the power-down registration request, the MS is powered off. However, upon failure to receive the response from the BS even after the attempts as many times as the maximum attempt sequence, the MS is automatically powered off. In this case, the BS does not recognize the power-off state of the automatically powered-off MS. In this state, upon receipt of a request for call setup to the powered-off MS from another MS, the BS attempts paging the powered-off MS through the paging channel. Undesirably, however, this gives an unnecessary load to the BS. Furthermore, compared with when the BS recognizes the power-off state of the MS, the call setup requesting MS's user should wait for a relatively longer time before receiving a voice mail service for the power-off state of the called MS.